1. Field of the Invention
The present invention relates to enhanced safety coaxial cables which resist excessive heating and possible burns resulting from undesired induced RF currents on the electrical cables.
2. Description of the Prior Art
The advantageous use of coaxial cables having an inner axially oriented elongated conductor separated from an annular electrically conductive shield by a dielectric material has long been known. Such coaxial cables have been used in magnetic resonance imaging, as well as numerous other uses.
Among the important safety concerns related to magnetic resonance imaging technology are the possible burns and excessive heat due to the induced RF currents on the electrical cables. To reduce the risk of such burns, the users of the MR scanners are instructed to minimize patient contact with cables. Such contact, however, is unavoidable in many cases such as when using ECG cables, surface coils, or intracavity coils.
To minimize burns and induced currents on the cables, some commercial MR coils, such as the magnetic resonance coils of GE Medical Systems, for example, use patient safety modules. See FIG. 1. This design decreases the unbalanced currents on the coaxial cable. In addition to patient safety, this design effects reduction in radiation losses and common mode noise in the coil.
The operating principle of the prior art design, shown in FIG. 1, is as follows: The signal inside the coaxial cable does not "see" the balun circuit. The balun circuit is effective only with respect to the currents flowing outside the coaxial cable as the unbalanced currents. The capacitor and the number turns of the coaxial cable is adjusted so that it resonates at the operating frequency. The circuit is encapsulated inside a "can" (or a metal box) with a ground connection at one end so that when an external RF pulse is applied, there will be no current induction in the balun circuit. The resonance circuit will behave as an open circuit for external RF signal and practically eliminates the unbalanced current flowing from cable to the coax. This circuit is an open circuit for the unbalanced current only. The common mode signal (the signal between the inner conductor and the shield of the coaxial cable) will not be affected by the presence of this circuit.
An extension of the above mentioned design is to use other kinds of balun circuits. In the ARRL Handbook for Radio Amateurs, some other balun circuit designs are shown. See R. Schetgen, "The ARRL Handbook for Radio Amateurs," Seventy-Second, The American Radio Relay League, Newington, Conn. (1995). This design is based on the balun (balanced unbalanced transformer) circuit. A typical balun circuit is shown in FIG. 2.
Similar and more serious problems exist for the coils that are inserted inside the body such as endorectal, esophageal, and intravascular RF probes. As these devices are closer to the body, the risk of burning a patient is increased. Also, the wavelength of the RF signal in the body is approximately nine times shorter as compared with the wavelength in the air. As a result, current induction on short cables is possible. There remains, therefore, a need for an improved coaxial cable which will perform effectively for its intended purpose while resisting undesired excessive heating or burning of a patient or those working with a system as a result of induced RF currents on the electrical cables.